1. Field of the Invention
The present invention relates to an electrostatic latent image developing toner used for developing an electrostatic latent image by the electrophotographic method, the electrostatic recording method, or the like, a production method thereof, an electrostatic latent image developer, and an image forming method.
2. Description of the Related Art
Methods for visualizing image information via a process of forming and developing an electrostatic latent image, such as the electrophotographic method, are currently utilized in various fields. Image formation by such methods is carried out by uniformly charging a photoreceptor surface, forming an electrostatic latent image by the exposure of the photoreceptor surface with a laser beam according to image information, then forming a toner image by developing the electrostatic latent image with a developer including a toner, and finally transferring and fixing the toner image onto a recording medium surface.
As a developer used in the electrophotographic method, a two-component developer made of a toner and a carrier, and a one-component developer using only a magnetic toner or a non magnetic toner are known. In general, a toner is produced by a kneading-pulverizing process, which consists of melt-compounding and cooling a thermoplastic resin with a pigment, a charge controlling agent and a releasing agent such as a wax, and then finely pulverizing and classifying. In order to improve the flowability and the cleaning property, the toner is used with inorganic particles or organic particles added to the surface of the toner particles.
In recent years, since images of higher image quality are demanded in the presentation of information documents created by various methods, increased image quality in various image forming methods has been much researched. Image forming methods utilizing the electrophotographic method are no exception in this regard. In the electrophotographic method in particular, a high function toner having a sharper particle size distribution with smaller diameter and the like is required in order to realize more high-definition images.
On the other hand, in recent years demand for energy saving has greatly increased also with respect to the electrophotographic method, and there is great demand for techniques to fix toner with reduced energy and toner that can be fixed at reduced temperature in order to reduce the amount of energy used in copying machines and toners. Conventionally, as a means for lowering the toner fixing temperature, a technique of lowering the glass transition temperature of the resin (binder resin) comprising the toner is generally known. However, by lowering the glass transition temperature, even though an excellent low temperature fixing property can be provided, aggregation (blocking) of the toner particles is easily generated, such that image quality defects such as white stripes, dropping, toner spilt stripes, or the like can be generated.
Therefore, in practical use, the lower limit value of the glass transition temperature of the binder resin used for a conventional toner is about 50° C. Additionally, the lowest fixing temperature in the case of using a toner using a binder resin having a 50° C. glass transition temperature is about 140° C., although this also depends on the kind of fixing machine. In such cases, a plasticizer can be used to further lower the lowest fixing temperature. However, in this case as in the case of lowering the glass transition temperature of the binder resin a problem arises in that the toner storage property deteriorates.
In order to solve these problems, a method for using a crystalline resin as the binder resin constituting the toner has long been known as a means for achieving both the blocking prevention and the low temperature fixing property (see, for example, Japanese Patent Application Publication (JP-B) Nos. 56-13943, 62-39428 and 63-25335). However, according to these techniques, since the melting point of the crystalline resin used is too low, there are problems in terms of the blocking property, insufficient fixing performance with respect to paper, and the like.
Therefore, for the purpose of improvement of the fixing property with respect to paper, a technique of using a crystalline polyester resin has been proposed. For example, a toner using a mixture of a non crystalline polyester resin and a crystalline polyester resin as the binder resin has been proposed (see JP-B No. 62-39428). However, according to technique, since the melting point of the crystalline polyester resin is high, the problem exists that a low temperature fixing property cannot be further improved on.
As a means of solving these problems, a technique of using a toner which is a mixture of a crystalline resin having a melting point of 110° C. or less and a non crystalline resin has been proposed (see JP-B No. 4-30014). However, in the case of mixing a non crystalline resin with a crystalline resin, the melting point of the toner is lowered and toner blocking generated, and thus the method is problematic in terms of practical use. Moreover, in the case that the ratio of the non crystalline resin component is high with respect to the crystalline resin component, since the characteristics of the non crystalline resin component are greatly reflected, it is difficult to provide a fixing temperature lower than that of the conventional toners. Additionally, since the glass transition temperature of the non crystalline resin is lowered, blocking property can deteriorate.
Furthermore, crystalline resins have low electric resistance due to the high degree of crystallization. Therefore, when an image is formed using a toner made of a crystalline resin, particularly in a high temperature high humidity environment, image defects such as injection superimposition and transfer failure are generated. Furthermore, since the toner is poor also in terms of bonding property with respect to paper, the strength of the image formed after fixation is also insufficient.
These problems cannot be improved even when a crystalline resin is used mixed with a non crystalline resin. That is, when the ratio of the crystalline resin in the binder resin used for the toner is high, even though the low temperature fixing property is excellent, the blocking resistance property, the image strength (bonding property with the paper) and the charging property (resistance) are poor. On the other hand, when the ratio of the non crystalline resin is high, even though the blocking resistance property, the image strength and the charging property (resistance) are improved, the low temperature fixing property, which is the most important property, is insufficient.
As mentioned above, a toner capable of realizing both a sufficient low temperature fixing property and storage property (blocking resistance property) has not been obtained.
On the other hand, with regard to the production method for a toner instead of the component materials of the toner as mentioned above, the toners commonly and widely used have conventionally been produced by the so-called kneading-pulverizing process (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 51-23354). This production method obtains a toner by melt-compounding a mixture of a binder resin and a coloring agent produced by various methods, and, as needed, a releasing agent, a charge controlling agent, a magnetic material, or the like, mixed by a dry process, and then pulverizing and classifying.
When producing a toner having an excellent low temperature fixing property by the kneading-pulverizing process, a binder resin having a low glass transition temperature needs to be included. However, since a molten and kneaded product including such a binder resin cannot be pulverized due to the absence of brittleness, and furthermore, the binder resin can fuse and adhere to the various kinds of production equipment such as piping and collection devices used in the production thereof. Therefore, a toner having an excellent low temperature fixing property cannot be produced industrially by the kneading-pulverizing process. The same applies when using a crystalline resin as the binder resin in view of, for example, the decline of the yield due to the difficulty of pulverizing of the molten and kneaded product.
On the other hand, when a non crystalline resin is used in a combination with a crystalline resin as the binder resin with a larger ratio of the non crystalline resin, since the non crystalline resin forms a continuous phase in the molten and kneaded product, it can be pulverized. However, since the melting characteristics of a toner of such composition depend on the non crystalline resin, it is difficult to realize a low temperature fixing property.
As described above, according to the conventional kneading-pulverizing process, it has been difficult to obtain a toner capable of realizing low temperature fixation in view of production methods.
Nevertheless, recently, production methods for toners, using various kinds of polymerization processes, which are different from the kneading-pulverizing process, have been proposed. For example, a preparation method for a toner by a suspension polymerization process, a preparation method by a dispersion polymerization process (see JP-A Nos. 62-073276, 5-027476), and a preparation method by an emulsification polymerization aggregation process have been proposed.
Among these production methods, even though toner particle size distribution can be improved to some extent by the suspension polymerization process or the dispersion polymerization process, since the particle size distribution cannot be improved dramatically compared with a toner obtained by the kneading-pulverizing process, it is disadvantageous in that a classifying operation is required in most cases.
On the other hand, the emulsion-polymerization aggregation process provides a sharp particle size distribution, and furthermore, it enables of controlling the toner shape from a spherical to a potato shape. Therefore, recently in particular, such toner has come to be used as the preferred toner in image forming apparatuses with inexpensive high quality cleaning systems, with widespread commercial availability.
The emulsion-polymerization aggregation process is a method for producing a toner by producing a dispersion of resin particles by a polymerization process such as emulsion polymerization, and also producing a coloring agent dispersion with a coloring agent dispersed in a solvent, mixing the dispersions, forming aggregate particles by aggregation of the above-mentioned resin particles and coloring agent to a desired particle size by heating, controlling the pH, and/or adding a flocculating agent or the like, then growing the aggregate particles to a desired particle size, and finally, heating and fusing the aggregate particles at a temperature equal to or higher than the glass transition temperature of the resin particles.
The advantage of the new production methods is that the degree of freedom in controlling the toner structure is high, which was not been achieved by the kneading-pulverizing process.
For example, for a toner to be used for the oil-less fixation, a releasing agent such as a wax is included therein. Here, when the particle size of a toner obtained by the conventional kneading-pulverizing process is reduced to realize high image quality, flowability is seriously deteriorated such that black stripes, dropping pollution, or the like are generated by soft blocking, or the concentration cannot be controlled due to deterioration of the toner dispensability, which is problematic. This is because a large amount of wax tends to exist on the surface of the toner obtained, since pulverization of the kneaded and molten product takes place at the interface with the wax phase dispersed in a matrix.
On the other hand, with a toner obtained by the new production method, since a structure for encapsulating a releasing agent, that is, a core shell structure where a core layer including a releasing agent is covered with a shell layer made of a binder resin, can be realized, deterioration of flowability or the like is not generated.
Many attempts for obtaining a toner having a low temperature fixing property utilizing the new production methods have been proposed (see, for example, JP-A No. 10-123748). Specifically, a toner with a core shell structure using a binder resin having a low glass transition temperature suitable for low temperature fixation as the core layer binder resin, and a binder resin having a relatively high glass transition temperature as the binder resin comprising the shell layer for covering the core layer, has been proposed.
In the toner having the core shell structure, since binder resins of different kinds and physical properties can be used for the core layer and the shell layer, each layer can easily bear a specific function independently. By making the toner structure a core shell structure, an effect of distributing two or more functions required for the toner to the core layer and the shell layer separately can be obtained (hereinafter, also referred to as the “function distributing effect”); however, in a toner with a single layer structure produced by the conventional kneading-pulverizing process, the function distributing effect cannot be obtained.
Therefore, in a toner with a single layer structure produced by the conventional kneading-pulverizing process, even when two kinds of binder resins having different glass transition temperatures are used, since they are present in the toner in a compatible state, a low temperature fixing property and good toner storage property in a high temperature environment cannot be realized. However, a toner having the core shell structure can easily realize both the low temperature fixing property and storage property.
However, in order to realize energy saving, fixing at a temperature lower than the conventional configuration (an ultra low temperature fixing property) is required in a toner. Moreover, since a process speed increase inevitably gives rise to substantial lowering of the fixing temperature, an ultra low temperature fixing property is also required in order to realize high speed. However, in a toner having the conventional core shell structure, even when simply the glass transition temperature of the binder resin material used for the core layer and the shell layer is reconsidered in order to secure a lower temperature fixing property, it has been difficult to realize both the ultra low temperature fixing property and sufficient storage property.